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20
General, Organic, and
Biochemistry, 8e
Bettelheim, Brown
Campbell, & Farrell
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20-1
20 Chapter 20
Carbohydrates
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20-2
20 Carbohydrates
• Carbohydrate: a polyhydroxyaldehyde or
polyhydroxyketone, or a substance that gives
these compounds on hydrolysis.
• Monosaccharide: a carbohydrate that cannot be
hydrolyzed to a simpler carbohydrate.
• Monosaccharides have the general formula CnH2nOn,
where n varies from 3 to 8.
• Aldose: a monosaccharide containing an aldehyde
group.
• Ketose: a monosaccharide containing a ketone group.
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20-3
20 Monosaccharides
• Monosaccharides are classified by their number
of carbon atoms.
Name
Formula
Triose
Tetrose
Pentose
C3 H6 O3
C4 H8 O4
Hexose
Heptose
Octose
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C5 H1 0 O 5
C6 H1 2 O 6
C7 H1 4 O 7
C8 H1 6 O 8
20-4
20 Monosaccharides
• There are only two trioses:
CHO
CH2 OH
CHOH
C= O
CH2 OH
CH2 OH
Glyceraldehyde
(an aldotriose)
D ihydroxyacetone
(a ketotriose)
• Often aldo- and keto- are omitted and these
compounds are referred to simply as trioses.
• Although “triose” does not tell the nature of the
carbonyl group, it at least tells the number of carbons.
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20-5
20 Monosaccharides
• Glyceraldehyde, the simplest aldose, contains a
stereocenter and exists as a pair of enantiomers.
CHO
CHO
H
C
OH
CH2 OH
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HO
C
H
CH2 OH
20-6
20 Monosaccharides
• Fischer projection: a two dimensional
representation for showing the configuration of
tetrahedral stereocenters.
• Horizontal lines represent bonds projecting forward
from the stereocenter.
• Vertical lines represent bonds projecting to the rear.
• Only the stereocenter is in the plane.
CHO
H
C
OH
CH2 OH
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con vert to
a Fischer
projection
CHO
H
OH
CH2 OH
20-7
20 D,L Monosaccharides
• In 1891, Emil Fischer made the arbitrary
assignments of D- and L- to the enantiomers of
glyceraldehyde.
CHO
H
OH
CHO
HO
H
CH2 OH
CH2 OH
D-Glyceraldehyde
L-Glyceraldehyde
[]25 = +13.5°
[]25 = -13.5°
D
D
• D-monosaccharide: the -OH on its penultimate carbon
is on the right in a Fischer projection.
• L-monosaccharide: the -OH on its penultimate carbon
is on the left in a Fischer projection.
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20-8
20 D,L Monosaccharides
• The most common D-tetroses and D-pentoses are:
CHO
H
OH
H
OH
CH2 OH
D-Erythrose
CHO
HO
H
H
OH
CHO
H
OH
H
OH
H
OH
CHO
H
H
H
OH
H
OH
CH2 OH
D-Threose
CH2 OH
CH2 OH
D-Ribose
2-Deoxy-D-ribose
• The three most common D-hexoses are:
H
HO
H
H
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CHO
OH
H
OH
OH
CH2 OH
D-Glucose
CHO
H
OH
HO
H
HO H
H OH
CH2 OH
D-Galactose
CH2 OH
C= O
HO
H
H OH
H OH
CH2 OH
D-Fructose
20-9
20 Amino Sugars
• Amino sugars contain an -NH2 group in place of
an -OH group.
• Only three amino sugars are common in nature: Dglucosamine, D-mannosamine, and D-galactosamine.
CHO
H N H2
HO H
H OH
H OH
CH2 OH
CHO
H 2N 2 H
HO H
H OH
H OH
CH2 OH
D-Glucosamine
D-Mannosamine
(C-2 stereois omer
of D-glucosamine
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CHO
H N H2
HO H
HO 4 H
H OH
CH2 OH
D-Galactosamine
(C-4 stereois omer
of D-glucosamine)
H
HO
H
H
CHO O
N HCCH 3
H
OH
OH
CH2 OH
N-Acetyl-Dglucos amine
20-10
20 Cyclic Structure
• Aldehydes and ketones react with alcohols to
form hemiacetals.
• Cyclic hemiacetals form readily when the hydroxyl and
carbonyl groups are part of the same molecule and
their interaction can form a five- or six-membered ring.
O
4
1
H
red raw to show
-OH an d -CHO
clos e to each oth er
O-H
4-Hyd roxypentanal
1
4
O
H
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C
H
O
H
O-H
O
A cyclic hemiacetal
20-11
20 Haworth Projections
• D-Glucose forms these cyclic hemiacetals.
1
CHO
H
OH
HO
H
H
H
red raw to sh ow th e -OH
on carbon-5 close to the
aldeh yd e on carbon-1
OH
5
OH
H
CH2 OH
D -Glucose
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CH 2 OH
5
OH
H
O
H
OH H C1
HO
H
CH2 OH
O OH (  )
H
H
OH H
HO
H
H OH
-D -Glucopyranose
(-D -Glucose)
OH
CH2 OH anomeric
carb on
OH
H
H
+
OH H
HO
OH(  )
H OH
-D -Glucopyranose
( -D -Glucos e )
20-12
20 Haworth Projections
• A five- or six-membered cyclic hemiacetal is
represented as a planar ring, lying roughly
perpendicular to the plane of the paper.
• Groups bonded to the carbons of the ring then lie
either above or below the plane of the ring.
• The new carbon stereocenter created in forming the
cyclic structure is called an anomeric carbon.
• Stereoisomers that differ in configuration only at the
anomeric carbon are called anomers.
• The anomeric carbon of an aldose is C-1; that of the
most common ketoses is C-2.
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20-13
20 Haworth Projections
•
In the terminology of carbohydrate chemistry,
•  means that the -OH on the anomeric carbon is on the
same side of the ring as the terminal -CH2OH.
•  means that the -OH on the anomeric carbon is on the
side of the ring opposite from the terminal -CH2OH.
• A six-membered hemiacetal ring is called a pyranose,
and a five-membered hemiacetal ring is called a
furanose.
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O
O
Furan
Pyran
20-14
20 Haworth Projections
• Aldopentoses also form cyclic hemiacetals.
• The most prevalent forms of D-ribose and other
pentoses in the biological world are furanoses.
HOCH2
H
H
O
H
HOCH2
H
OH ()
O
H
H
OH ()
H
H
OH
OH
OH
H
-2-D eoxy-D -ribofuranose
-D -Ribofuranose
(-2-D eoxy-D -rib os e)
(-D -Rib os e)
• The prefix “deoxy” means “without oxygen.”
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20-15
20 Haworth Projections
• D-Fructose (a 2-ketohexose) also forms a fivemembered cyclic hemiacetal.
HOCH2
5
1
O
H HO
CH2 OH
2
OH( )
H
HO
H
 -D -Fructofuranose
( - D -Fructos e)
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1
2
CH2 OH
C=O
HO
H
H
OH
H 5 OH
CH2 OH
D -Fru ctose
HOCH2
5
O
H HO
H
OH ( )
2
CH2 OH
HO
H
1
 - D -Fru ctofu ran os e
(- D -Fructose)
20-16
20 Chair Conformations
• For pyranoses, the six-membered ring is more
accurately represented as a chair conformation.
HO
HO
CH2 OH
O
anomeric
carbon
OH()
OH
 -D -Glu copyran os e
( - D -Glucos e)
HO
HO
CH2 OH
OH
O
C
OH H
D -Glucos e
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HO
HO
CH2 OH
O
HO
OH( )
- D -Glu copyran os e
(  - D -Glucose)
20-17
20 Chair Conformations
• In both Haworth projections and chair conformations,
the orientations of groups on carbons 1- 5 of -Dglucopyranose are up, down, up, down, and up.
6
CH2 OH
5
O OH()
H
H
4 OH
1
H
HO
H
3
2
H OH
-D -Glucop yranose
(Haw orth p rojection)
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6
CH2 OH
4
HO
HO
O
5
3
2
OH 1
OH( )
 - D -Glucopyranose
(ch air con formation)
20-18
20 Mutarotation
• Mutarotation: the change in specific rotation that
accompanies the equilibration of - and anomers in aqueous solution.
• Example: when either -D-glucose or -D-glucose is
dissolved in water, the specific rotation of the solution
gradually changes to an equilibrium value of +52.7°,
which corresponds to 64% beta and 36% alpha forms.
HO
HO
CH2 OH
O
OH
OH
-D -Glucopyranose
[] D 2 5 = + 18.7°
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HO
HO
CH2 OH
OH
O
C
HO
H
Open-chain form
HO
HO
CH2 OH
O
HO
OH
-D -Glucopyranose
[] D 2 5 = +112°
20-19
20 Physical Properties
• Monosaccharides are colorless crystalline solids,
very soluble in water, but only slightly soluble in
ethanol
• Sweetness relative to sucrose:
S w eetness
Relative to
Carbohydrate
S ucrose
fructos e
1.74
sucrose (tab le sugar) 1.00
honey
0.97
glu cose
0.74
maltose
0.33
galactos e
0.32
lactose (milk su gar)
0.16
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S w eetness
Relative to
Artificial
Sw eetener
S ucrose
saccharin
450
acesu lfame-K
200
aspartame
180
20-20
20 Formation of Glycosides
• Treatment of a monosaccharide, all of which exist
almost exclusively in cyclic hemiacetal forms,
with an alcohol gives an acetal.
anomeric
carbon
CH2 OH
O OH
H
+
H
H
+ CH3 OH
OH H
-H2 O
HO
H
glycos idic
H OH
CH2 OH
bond
-D -Glu copyran os e
O OCH3
H
(-D -Glu cose)
H
+
OH H
H
HO
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CH2 OH
OH
H
H
OH H
HO
OCH3
H OH
H OH
Methyl -D -glu copyran os ide Methyl -D -glu copyran os ide
(Methyl -D -glu coside)
(Methyl -D -glucos ide)
20-21
20 Formation of Glycosides
• A cyclic acetal derived from a monosaccharide is called
a glycoside.
• The bond from the anomeric carbon to the -OR group is
called a glycosidic bond.
• Mutarotation is not possible in a glycoside because an
acetal, unlike a hemiacetal, is not in equilibrium with
the open-chain carbonyl-containing compound.
• Glycosides are stable in water and aqueous base, but
like other acetals, are hydrolyzed in aqueous acid to an
alcohol and a monosaccharide.
• Glycosides are named by listing the alkyl or aryl group
bonded to oxygen followed by the name of the
carbohydrate in which the ending -e is replaced by -ide.
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20-22
20 Reduction to Alditols
• The carbonyl group of a monosaccharide can be
reduced to an hydroxyl group by a variety of
reducing agents, including NaBH4 and H2 in the
presence of a transition metal catalyst.
• The reduction product is called an alditol.
HO
HO
CH2 OH
O
OH
OH
-D -Glucop yranose
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CHO
H OH
HO H
NaBH4
H OH
H OH
CH2 OH
D -Glu cose
CH2 OH
H OH
HO H
H OH
H OH
CH2 OH
D -Glucitol
(D -Sorbitol)
20-23
20 Reduction to Alditols
• Sorbitol is found in the plant world in many berries and
in cherries, plums, pears, apples, seaweed, and algae.
• It is about 60 percent as sweet as sucrose (table sugar)
and is used in the manufacture of candies and as a
sugar substitute for diabetics.
• These three alditols are also common in the biological
world.
CH2 OH
CH2 OH
H
OH
H
OH
CH2 OH
Erythritol
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HO
HO
H
H
H
H
OH
OH
CH2 OH
D -Mannitol
CH2 OH
H
OH
HO
H
H
OH
CH2 OH
Xylitol
20-24
20 Oxidation to Aldonic Acids
• The aldehyde group of an aldose is oxidized under
basic conditions to a carboxylate anion.
• The oxidation product is called an aldonic acid.
• A carbohydrate that reacts with an oxidizing agent to
form an aldonic acid is classified as a reducing sugar
(it reduces the oxidizing agent).
O
H
C
HO
HO
CH2 OH
O
OH
- D-Glucopyranose
(- D-Glucose)
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OH
H
HO
H
H
OH oxidizing
agent
H
OH
basic
OH solution
CH2 OH
D-Glucose
O-
O
C
H
HO
H
H
OH
H
OH
OH
CH2 OH
D-Gluconate
20-25
20 Oxidation to Uronic Acids
• Enzyme-catalyzed oxidation of the primary
alcohol at C-6 of a hexose yields a uronic acid.
• Enzyme-catalyzed oxidation of D-glucose, for example,
yields D-glucuronic acid.
CHO
enzymeH
OH
catalyzed
HO
H
oxidation
H
OH
H
OH
CH2 OH
D-Glucose
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CHO
H
OH
COOH
O
HO
H
HO
H
OH
HO
OH
H
OH
COOH
D-Glucuronic acid
(a uronic acid)
OH
20-26
20 D-Glucuronic Acid
• D-Glucuronic acid is widely distributed in the plant and
animal world.
• In humans, it is an important component of the acidic
polysaccharides of connective tissues.
• It is used by the body to detoxify foreign phenols and
alcohols; in the liver, these compounds are converted
to glycosides of glucuronic acid and excreted in the
urine.
COOHO
HO
HO
O
O
OH
Propofol
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A u rin e-s olu ble glucuronide
20-27
20 Phosphate Esters
• Mono- and diphosphoric esters are intermediates
in the metabolism of monosaccharides.
• For example, the first step in glycolysis is conversion
of D-glucose to -D-glucose 6-phosphate.
• Note that at the pH of cellular and intercellular fluids,
both acidic protons of a diphosphoric ester are ionized,
giving it a charge of -2.
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CHO
H
OH
HO
H
H
OH
H
OH O
CH2 O-P- O OD-Glucose 6-phosphate
O
O P OO
CH2
HO
HO
O
HO
OH
-D-Glucose 6-phosphate
20-28
20 Disaccharides
• Sucrose (table sugar)
• Sucrose is the most abundant disaccharide in the
biological world; it is obtained principally from the juice
of sugar cane and sugar beets.
• Sucrose is a nonreducing sugar.
CH2 OH
O
OH
1
HO
HO
OH
HO
OH
O
O
HO 2
CH2 OH
1
OH
HOCH2
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a unit of -Dglucopyranose
CH2 OH
O
HOCH2
O
HO
1
O
2
-1,2-glycosidic bond
a unit of -Dfructofuranose
CH2 OH
OH
1
20-29
20 Disaccharides
• Lactose
• Lactose is the principal sugar present in milk; it makes
up about 5 to 8 percent of human milk and 4 to 6
percent of cow's milk.
• It consists of D-galactopyranose bonded by a -1,4glycosidic bond to carbon 4 of D-glucopyranose.
• Lactose is a reducing sugar.
CH2 OH
OH
O
CH2 OH
O
OH
4
1
OH
CH2 OH
-1,4-glycosid ic bond
O
4
O
OH
OH
OH
HO
1
OH
O
HO
CH2 OH
O
OH
OH
OH
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20-30
20 Disaccharides
• Maltose
• Present in malt, the juice from sprouted barley and
other cereal grains.
• Maltose consists of two units of D-glucopyranose
joined by an -1,4-glycosidic bond.
• Maltose is a reducing sugar.
1
HOCH2 O
HO
CH2 OH
4
O
OH
OH
HO
OH
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HO
O OH HO
-1,4-glycosidic
bond
CH2 OH
O
1
OH 4 CH2 OH
O
O
OH
HO
OH
20-31
20 Polysaccharides
• Polysaccharide: a carbohydrate consisting of
large numbers of monosaccharide units joined by
glycosidic bonds.
• Starch: a polymer of D-glucose.
• Starch can be separated into amylose and amylopectin.
• Amylose is composed of unbranched chains of up to
4000 D-glucose units joined by -1,4-glycosidic bonds.
• Amylopectin contains chains up to 10,000 D-glucose
units also joined by -1,4-glycosidic bonds; at branch
points, new chains of 24 to 30 units are started by 1,6-glycosidic bonds.
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20-32
20 Polysaccharides
• Figure 20.3 Amylopectin.
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20-33
20 Polysaccharides
• Glycogen is the energy-reserve carbohydrate for
animals.
• Glycogen is a branched polysaccharide of
approximately 106 glucose units joined by -1,4- and 1,6-glycosidic bonds.
• The total amount of glycogen in the body of a wellnourished adult human is about 350 g, divided almost
equally between liver and muscle.
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20-34
20 Polysaccharides
• Cellulose is a linear polysaccharide of D-glucose
units joined by -1,4-glycosidic bonds.
• It has an average molecular weight of 400,000 g/mol,
corresponding to approximately 2200 glucose units per
molecule.
• Cellulose molecules act like stiff rods and align
themselves side by side into well-organized waterinsoluble fibers in which the OH groups form numerous
intermolecular hydrogen bonds.
• This arrangement of parallel chains in bundles gives
cellulose fibers their high mechanical strength.
• It is also the reason why cellulose is insoluble in water.
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20-35
20 Polysaccharides
• Figure 20.4 Cellulose is a linear polymer
containing as many as 3000 units of D-glucose
joined by -1,4-glycosidic bonds.
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20-36
20 Polysaccharides
• Cellulose (cont’d)
• Humans and other animals cannot use cellulose as
food because our digestive systems do not contain glucosidases, enzymes that catalyze hydrolysis of glucosidic bonds.
• Instead, we have only -glucosidases; hence, the
polysaccharides we use as sources of glucose are
starch and glycogen.
• Many bacteria and microorganisms have glucosidases and can digest cellulose.
• Termites have such bacteria in their intestines and can
use wood as their principal food.
• Ruminants (cud-chewing animals) and horses can also
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digest grasses and hay.
20-37
20 Acidic Polysaccharides
• Acidic polysaccharides: a group of
polysaccharides that contain carboxyl groups
and/or sulfuric ester groups, and play important
roles in the structure and function of connective
tissues.
• There is no single general type of connective tissue.
• Rather, there are a large number of highly specialized
forms, such as cartilage, bone, synovial fluid, skin,
tendons, blood vessels, intervertebral disks, and
cornea.
• Most connective tissues are made up of collagen, a
structural protein, in combination with a variety of
acidic polysaccharides.
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20-38
20 Acidic Polysaccharides
• Hyaluronic acid
• contains from 300 to 100,000 repeating units.
• is most abundant in embryonic tissues and in
specialized connective tissues such as synovial fluid,
the lubricant of joints in the body, and the vitreous of
the eye where it provides a clear, elastic gel that
maintains the retina in its proper position
D -glucu ronic acid
N-Acetyl-D -glu cosamine
-
4
HO
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COO
4
O HO
O
1
CH2 OH
O
1
NH
C
H3 C
O
The rep eating unit of h yalu ronic acid
3
OH
O
3
20-39
20 Acidic Polysaccharides
• Heparin: a heterogeneous mixture of variably
sulfonated polysaccharide chains, ranging in
molecular weight from 6,000 to 30,000 g/mol.
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20-40
20 Acidic Polysaccharides
• Heparin (cont’d)
• Heparin is synthesized and stored in mast cells of
various tissues, particularly the liver, lungs, and gut.
• The best known and understood of its biological
functions is its anticoagulant activity.
• It binds strongly to antithrombin III, a plasma protein
involved in terminating the clotting process.
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20-41
20 Carbohydrates
End
Chapter 20
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20-42